An important component of integrated circuitry design and manufacturing is the testing and troubleshooting of an integrated circuit. Typically testing involves attempts to analyze internal, electrical signals. The analysis of the signals may be used to inspect circuit performance or to perform failure analysis of the integrated circuit.
One class of integrated circuits includes an active region (e.g., transistors) fabricated in a substrate. In particular, the active region resides close to the top surface of the integrated circuit. Traditional attempts to access signals internal to an integrated circuit often involve use of a Focused Ion Beam (FIB) directly from the top of the integrated circuit for accessing the signals. However, conventional attempts at utilizing FIB approaches for flip chips present significant difficulties.
A flip chip is an integrated circuit in which the active area of the chip is “flipped over” facing downward. Instead of facing up and bonded to the package leads with wires from the outside edges of the chip, the surface of the active area of the flip chip can be used for interconnection, which is typically done through metal bumps of solder. These bumps are soldered onto the package substrate or the circuit board itself and underfilled with epoxy. The flip chip allows for a large number of interconnects with shorter distances than wire, which greatly reduces inductance.
For flip chip devices it is inherently difficult to inspect circuit performance or analyze on-die signals for failure due to limited access to the top of the die. The top of the die is mounted to the package substrate such that the active region of the flip chip device is inaccessible from the top. Attempts to access the active region of the flip chip device by drilling through the underside die substrate is difficult. Typically, the die substrate is significantly thicker than the active region of the integrated circuit. For example, a die substrate may be 700 micrometers (μm), while an active region may be closer to 10 μm. To probe a line of the active region of a flip chip, the FIB must drill a sizeable hole through the die substrate. Drilling through the die substrate requires a significantly greater amount of effort, due to the thickness of the die substrate. Furthermore, since a FIB process is typically very expensive to run, drilling through the underside die substrate is also usually very costly. Moreover, the FIB probe point must be placed very accurately, due to the thickness of the die substrate.